The present invention relates to a hydraulic continuously variable speed transmission comprising a constant displacement hydraulic pump and a variable displacement hydraulic motor, and more particularly to a continuously variable speed transmission provided with a direct clutch for blocking a hydralic closed circuit, through which the pump is hydraulically connected to the motor, to render the both in a direct connecting state when a speed reduction ratio of the variable speed transmission has become 1.
Continuously variable speed transmissions have conventionally been proposed in versatile Japanese Official Gazette such as Japanese Patent Publication No. 32(1957)-7159, Japanese Patent Publication No. 56(1981)-50142 and the like, wherein a constant displacement hydraulic pump is connected to an input shaft, oil delivered from the pump is fed to a variable displacement hydraulic pump through a closed circuit, and the hydraulic pump is driven to drive an output shaft connected thereto.
It is known to provide a direct clutch which is capable of blocking the above hydraulic closed circuit, whereby the direct clutch device blocks the hydraulic closed circuit to integrally rotate the pump and the motor when the swash plate angle for variably controlling the displacement of the hydraulic motor is minimized (the speed reduction ratio of the variable speed transmission becomes 1.)
A method for controlling a continuously variable speed transmission having a direct clutch device, as disclosed in Japanese Patent laid-open Publication No. 54(1979)-134252 and Japanese Patent laid-open Publication No. 55(1980)-14312, comprises controlling the speed reduction ratio in such a way that the engine speed coincides with a reference engine speed corresponding to the throttle opening to obtain minimum fuel consumption and blocking the closed circuit by the direct clutch device to integrally rotate the pump and the motor when the speed reduction ratio becomes minimum or 1.
In controlling the direct clutch, as described above, when the direct clutch blocks or opens the closed circuit (when it is switched ON or OFF) at a speed reduction ratio of 1, the load of the engine is changed because of change of the hydraulic thrust forces acting on the motor plungers and change of the volumetric efficiency. Therefore, changes of the thrust force and the efficiency caused by ON/OFF switching of the direct clutch may produce changes of engine load and engine speed which lead to jerky traveling characteristics of the vehicle and deteriorate drive feelings.
Further, if the circuit is blocked by the direct clutch after speed reduction ratio has become "1" as disclosed in Japanese laid-open Patent Publication No. 55(1980)-1209, the actuation of the direct clutch may be delayed because it requires a certain amount of time to block the circuit completely by the direct clutch unit.